1. Field of the Invention
The present invention relates to a gas transfer machine for use as a fan, a vacuum pump, a compressor, or the like for transferring a gas based on the rotation of a rotor, and more particularly to a motor for rotating such a rotor.
2. Description of the Related Art
FIGS. 1 and 2 of the accompanying drawings show a conventional gas-transfer machine for use as a fan, a vacuum pump, a compressor, or the like for transferring a gas. As shown in FIGS. 1 and 2, the conventional gas transfer machine has an induction motor for rotating a rotatable member such as an impeller. Specifically, the conventional gas transfer machine includes a pump rotor 1, a motor rotor 7 directly coupled to the pump rotor 1 by a main shaft 2, and a stator 6 disposed around the motor rotor 7 and having windings 8. When the motor rotor 7 is rotated by a revolving magnetic field generated by the stator 6, the pump rotor 1 is rotated to transfer a gas from a gas inlet 30 to a gas outlet 31. The main shaft 2 is rotatably supported by bearings 3a, 3b. The stator 6 is covered with a can 11 that comprises a thin sheet of metal. The can 11 fully shields the stator 6 for protection against exposure to the gas, which may be corrosive, that may possibly be introduced through the bearing 3b. The motor rotor 7 and the stator 6 make up an induction motor. The motor rotor 7 has secondary conductors 9 in the form of conductive rods of aluminum or the like and end rings 10 joining the ends of the secondary conductors 9.
The conventional gas transfer machine shown in FIGS. 1 and 2 suffers the following problems: While the vacuum pump or compressor as the gas transfer machine is in operation, the gas discharged therefrom is heated when compressed, thereby heating the pump rotor to a high temperature. As a result, the motor rotor that is directly connected to the main shaft is also heated to a high temperature. Since the motor rotor rotates at a high speed, its mechanical strength needs to be taken into account for reliable operation thereof. Such a requirement makes it difficult to design the motor rotor to rotate at a higher speed. However, there is a demand for vacuum pumps and compressors whose motor rotors rotate at higher speeds for higher performance.
Since the induction motor has the secondary conductors in its rotor, the end rings and the secondary conductors pose a strength problem when the rotor rotates at high speeds. The end rings and the secondary conductors are also under thermal stresses at high temperatures. Furthermore, the efficiency of the induction motor cannot be increased because of a loss that is necessarily caused by the secondary conductors and the metal can.
Furthermore, the dimension of coil ends of the induction motor is not minimum because the induction motor usually employs a distributed winding pattern. Consequently, the motor has relatively large dimensions, which make the overall size of the gas transfer machine large.
Another problem of the induction motor is that it causes slippage. The actual rotational speed of the rotor of the induction motor cannot be determined from a power supply system, but needs to be detected by a sensor or the like coupled with the rotor.
The motor rotor that is held in contact with the gas being transferred comprises a composite material of electromagnetic sheet steel and aluminum. Therefore, if the gas being transferred is corrosive, then the motor rotor would be corroded. It is necessary to apply a corrosion-resistant coating to the motor rotor or cover the motor rotor with a can. However, the corrosion-resistant coating on the composite material of electromagnetic sheet steel and aluminum is not reliable and tends to be peeled off easily. The can that covers the motor rotor reduces the efficiency because it widens the gap between the motor rotor and the stator.
It is therefore an object of the present invention to provide a gas transfer machine which is of a compact structure, is stable in operation even when handling a corrosive gas, and can operate at a high speed.
According to the present invention, there is provided a gas transfer machine comprising a pump rotor mounted on a rotatable shaft for transferring a gas, and a reluctance-type motor for rotating the rotatable shaft about its own axis, the reluctance-type motor comprising a stator, a motor rotor surrounded by the stator, and a shield member isolating the stator from the motor rotor, the motor rotor being directly coupled to the rotatable shaft and having a plurality of magnetic salient poles.
Since the reluctance-type motor has no secondary conductors and end rings on the motor rotor, the motor rotor is of increased mechanical strength upon rotation at a high speed, and has increased efficiency as it causes no current loss. The stator can have concentrated windings on salient poles for minimizing coil ends and hence for reducing the size of the motor.
The shield member may comprise a molded body of synthetic resin having a surface positioned radially inwardly of an inner circumferential surface of the stator, the stator being embedded in the molded body of synthetic resin. The stator can thus be protected by the shield member against contact with the gas, which may be corrosive, so that windings of the stator can be protected. In addition, the molded body of synthetic resin causes no eddy current loss.
Alternatively, the shield member may comprise a can of synthetic resin or nonconductive material. The can of synthetic resin or nonconductive material is also effective to protect the stator and its windings against contact with the gas, which may be corrosive, and causes no current loss.
The motor rotor may have a plurality of permanent magnets disposed respectively in the magnetic salient poles. The permanent magnets in the magnetic salient poles can increase the torque generated by the motor with an increased output power thereof.
The motor rotor is preferably made of permalloy. The motor rotor of permally is resistant to corrosion due to contact with the gas, which may be corrosive, and also provides desired magnetic properties for the motor.